A power amplifier operates by converting DC supply power into radio frequency (RF) energy that is used to amplify an input RF signal. The efficiency at which a power amplifier is able to convert the DC power into RF energy is maximized at or near the power amplifier's saturation point (i.e. the nearer the power level of the RF input signal is to the saturation point, the higher the amplifier's efficiency will be). For this reason, it is desirable to operate a power amplifier as close to its saturation point as possible.
When an RF input signal has a low peak-to-average power ratio (i.e. when the power level of the RF input signal has little variation), a power amplifier can be operated near the saturation point since the peaks in the input power do not cause the amplifier to operate past the saturation point. In contrast, when an RF input signal has a high peak-to-average power ratio (i.e. when the power level of the RF input signal varies substantially), it is necessary to operate the power amplifier at a significant backoff, meaning at a reduced power level that ensures that the peaks in the input power do not cause the amplifier to operate past its saturation point. As a power amplifier is operated beyond its saturation point, the amount of distortion introduced into the output signal increases dramatically due to clipping. In addition to the loss of data caused by the clipping, because the clipping is a non-linear process, it introduces a significant amount of intermodulation distortion to the output signal.
FIG. 1 illustrates the concept of operating a power amplifier with backoff. FIG. 1 depicts the output power vs. the input power for an example power amplifier. As shown, any portion of an input signal that is amplified while the amplifier is operating in saturation will receive little or no amplification resulting in the clipping of the input signal. To minimize this clipping and the resultant distortion introduced by clipping, the power level of the input signal will be controlled (e.g. reduced) so that the entire signal will be amplified within the linear region of the amplifier. FIG. 1 represents a scenario where an input signal with a high peak-to-average power ratio is amplified. To ensure that the peaks of the input signal do not drive the amplifier into saturation, the power level of the input signal is controlled so that the peak power level is below the saturation point. Because of this, only small portions of the input signal (e.g. those that are at the peak power level as shown by the vertical line for the peak power level) are amplified within the amplifier's most efficient range. The majority of the input signal (represented by the vertical line for the average power level) is amplified at a lower efficiency.
In many cases, it is not desirable or possible to operate an amplifier completely within its linear region (i.e. with an adequate output backoff to ensure that the peak levels of the input signal do not cause the amplifier to go into saturation). This may be the case with multicarrier signals that exhibit very high peak-to-average power ratios. In such cases, the peak power levels of the input signal will extend into the saturation region of the amplifier. In reference to FIG. 1, this can be visualized as the vertical line for the peak power being shifted to the right into the saturation region.
To address these situations where the high peak-to-average power ratio of an input signal causes an amplifier to extend into saturation, various approaches have been created to account for the intermodulation distortion introduced into the output. These approaches involve introducing pre-distortion into the input signal that is intended to offset the distortion that will be caused when the input signal is amplified. More specifically, any portion of the signal that is amplified in the saturation region will experience clipping. This clipping introduces intermodulation distortion to the output signal. These approaches account for the intermodulation distortion added during amplification by employing appropriate circuitry and/or logic prior to amplification that add pre-distortion to the input. The pre-distortion then offsets the distortion added during amplification. To implement such approaches, either the nonlinear transfer characteristics of the amplifier must be known a priori or a monitoring scheme must be used to analyze the output signal to dynamically determine the distortion so that the appropriate pre-distortion can be applied to the input signal. In other words, these approaches attempt to cancel intermodulation distortion experienced in a power amplifier by predicting the intermodulation distortion and pre-compensating the signal with the inverse intermodulation distortion.
The above discussion assumes that the amplifier is being operated with a linearizer. A linearizer pre-distorts the input signal and increases its peak-to-average power ratio in a way that is exactly compensated by the amplifier. The use of a linearizer therefore serves to extend the linear range of the amplifier up to the point of clipping.
Additionally, the phase of an amplifier is also dependent on the power of the input signal. Therefore, as the power level of the input signal increases, more phase distortion will be introduced during amplification. A linearizer can also be used to account for such distortion by pre-distorting the phase of the input signal so that the input signal is amplified with a constant phase shift up to the point of clipping.
A linearizer can therefore be used to minimize distortion by using pre-distortion to yield a linear response up to the point of clipping. However, even with the use of a linearizer, an input signal with a high peak-to-average power ratio will still require operation at significant output backoff to avoid distortion that would be generated by clipping.